This invention relates to a method of and an apparatus for combusting lean concentrations of a burnable gas at superatmospheric pressure, and more particularly to such a combustion arrangement having a heat sink/pressure equalization chamber for protecting the combustor from back pressure generated during the combustion process.
U.S. Pat. No. 3,229,746 (the ""746 patent), which is incorporated by reference herein in its entirety, shows a heat recovery apparatus and a method suitable for combusting lean concentrations of a burnable gas. That patent, by way of example, is directed to the burning of lean gases such as, but not limited to, catalytic cracking off gas containing carbon monoxide concentrations of less than 8%. The invention in that patent permits the stabilizing of carbon monoxide ignition at a temperature in the range of from 1200xc2x0 F. to 1500xc2x0 F. After start-up, this temperature can be maintained in most cases by the combustion of the carbon monoxide alone. In the remaining cases, there is a minimal auxiliary fuel requirement to assure safe ignition and/or to maintain the desired amount of heat recovery.
FIG. 1 of this application shows the heat recovery apparatus of the ""746 patent. In FIG. 1, a setting generally designated by reference numeral 1 defines a combustion zone 2 and a heat recovery zone 3 horizontally disposed at grade level. Gas is communicated to the combustion zone 2 via gas chamber 4 and gas ports 6. Air is introduced via air chamber 7. The air enters the combustion chamber 2 through air ports 8. Secondary air to support the combustion of auxiliary fuel is admitted to the combustion chamber through conduits 9. The gas and air are intermittently commingled by opposing vortexes indicated by directing arrows 11 and 12 created by an aiming device shown as inclined conduits 13, which conduct the gas mixture from the gas chamber 4 to the gas ports 6 and short air pipes 14. Auxiliary burners 16 are provided to initially heat the gases in the combustion zone 2 to a suitable kindling temperature. Refractory material 17 lines the combustion zone 2 to re-radiate heat to the gases therein.
By the arrangement shown in FIG. 1, lean gas such as carbon monoxide in concentrations of less than 8%, such as catalytic cracking off gas, can be burned. Higher concentrations, of course, can be combusted more easily. An outstanding feature of the design shown in FIG. 1 is that it requires less than 1% of excess oxygen as measured in the products of combustion.
In the combustion chamber 2, a temperature in the range of from 1200xc2x0 F. to 1500xc2x0 F. can conveniently be maintained so that after light-off, carbon monoxide will usually be able to burn without the need for auxiliary fuel. Heat is liberated by the burning of carbon monoxide in the combustion zone 2. Intrinsically, there is a heat liberating system operating in the combustion chamber 2. In an extrinsic sense, the combustion zone 2 has been designed so that there is practically no heat input or heat removal to the combustion zone 2 vis-a-vis its surroundings. In particular, no cooling devices, such as heat exchange tubes, are associated with the combustion zone 2.
An end wall 18 is defined by a partition 19. Air ports 8 and gas ports 6 penetrate the partition 19 to define substantially concentric angular groups in the end wall 18. FIG. 1 shows an open checker brick wall 21 as a canalizing device, which causes the combustion gases to flow through restricted canals 22 to thereby increase commingling. The heat recovery zone 3 is defined by the setting 1 downstream of the combustion zone 2. An appropriate heat recovery apparatus, such as steam tubes, an economizer, a superheater, other fluid streams, and the like, can be provided in the heat recovery zone 3.
The setting 1 defines an enclosure for the combustion zone 2 having end walls and side walls extending between the end walls. All of the walls are arranged to re-radiate heat to the combustion zone via the refractory material 17. The exhaust port by which the hot gases are transmitted to the heat recovery zone 3 is at an end of the setting 1, opposite from the gas ports 6 and air ports 8, and constitutes a sufficiently small portion of one of the side walls to maintain re-radiation of heat from all walls of the enclosure at the highest level possible.
In addition, as shown in FIG. 1, the heat recovery zone 3, which is the only heat sink structure of the apparatus, is completely removed from exposure to the combustion zone 2. This is in comparison to conventional carbon monoxide boiler installations where a heat sink in the form of water tubes either is in the combustion zone or is exposed to radiant heat of the burning gases. Such an internal heat sink increases the requirement for auxiliary fuel and reduces to a marked extent flame stability and reliability of carbon monoxide gas conversion.
The apparatus shown in FIG. 1 typically operates at high temperatures. For example, the typical lean gas is fed to the apparatus at 600xc2x0 F. to 1100xc2x0 F. or higher. As a result of the combustion process, the combusted gases exiting the combustion zone can be in the range of 1200xc2x0 F. to 1800xc2x0 F. or higher.
FIGS. 2 and 3 show prior art apparatuses that adequately avoid overheating of the external casing plates thereof, which are respectively insulated on the lean gas chamber and the combusted gas chamber, by using a flow of pressurized ambient (xe2x80x9ccoldxe2x80x9d) air to an air chamber, which is formed and contained by these chambers. In such arrangements, the pressurized ambient air is utilized as the oxidant source to combust both the lean gas and an auxiliary fuel stream in the apparatus.
FIG. 2 shows a conventional combustion device 200, which includes a lean gas chamber 212, a combustor 230, a heat recovery zone 240, and an exhaust 250. Ambient air is pressurized and fed by an air pump 220 through a supply line 221 to the combustor 230. Lean gas 210 is supplied through a supply line 211 to the lean gas chamber 212.
FIG. 3 shows in more detail a combustion device 300. The combustion device 300 includes a lean gas chamber 312 and a combustor 330. Lean gas from lean gas chamber 312 enters the combustor 330 through a gas port 317. Pressurized ambient air 320 enters the combustor 330 through an air port 327. The combustion device 300 is insulated by a refractory lining 301. Combustion products exit the combustor 330 and are sent to a heat recovery section 340, typically through a heat exchanger (not shown).
One having ordinary skill in the art will appreciate that a suitable number of auxiliary burners 16 (shown in FIG. 1) may be provided as start-up means to initially heat the gases in the combustor 230 (FIG. 2) or 330 (FIG. 3) to a desired kindling temperature, or as a means to provide a level of heat input for the desired heat recovery.
As discussed above, such apparatuses are most typically used in processes where the lean gas is delivered to the apparatus at some pressure above atmospheric pressure (for example, 0.1 psig to 5.0 psig or higher), and the combusted gases typically are discharged to the atmosphere after heat recovery and, in some instances, after exhaust gas clean-up systems. This, however, results in a back pressure within the combustion zone. As noted in FIG. 2, air is supplied to the apparatus with a pump to meet the pressure requirements. The apparatus, of course, is designed to contain and withstand these internal pressures. The advantage of the configuration of such an apparatus is the economics of its construction for the pressure containment discussed above, resulting from integration of the gas chamber and air chamber within the overall pressure container. Thus, only nominal pressure differentials exist between the respective chambers.
We have found, however, that a problem arises in the conventional arrangement shown in FIG. 3. In that embodiment, the ambient air is used to cool the refractory lining 301 of the combustor 330. Thus, the lean gas chamber 312 and the combustor 330 are in contact with the ambient air 320. Nevertheless, some applications require that the ambient air 320 be preheated prior to combustion. When this occurs, the temperature of the ambient air 320 will no longer be sufficient to cool the refractory lining 301. In turn, problems arise in cooling the chambers, resulting in expansion and structural instability of those chambers.
Indeed, we have found that, for process and energy conservation reasons, the air supply should be preheated, to about 200xc2x0 F. to 600xc2x0 F. or higher. In these cases we have found that the air chamber containment as previously defined will no longer provide sufficient cooling to avoid technical problems. Accordingly, a need has arisen to provide a superatmospheric combustion device having an internal heat sink/pressure equalization chamber.
An object of this invention is to provide for the use of preheated combustion air, while retaining the construction advantages of minimal pressure differentials between the respective gas and air chambers.
Another object of this invention is to provide a superatmospheric combustion device having an internal heat sink/pressure equalization chamber. Yet another object of the invention is to provide such a combustion device for use with preheated combustion air.
In one aspect, the present invention provides a method of operating a superatmospheric combustion device. The method includes providing a superatmospheric combustion device, which includes a lean gas chamber, a combustor, a heat recovery section, and an exhaust, feeding lean gas to the lean gas chamber, providing a heat sink/pressure equalization chamber and a preheated air chamber within the combustion device, feeding pressurized ambient air to the heat sink/pressure equalization chamber, feeding preheated air to the preheated air chamber, exchanging heat from the lean gas chamber, the preheated air chamber, and the combustor to the pressurized ambient air in the heat sink/pressure equalization chamber, feeding the lean gas from the lean gas chamber to the combustor, feeding the preheated air from the preheated air chamber to the combustor, and combusting the lean gas and the preheated air in the combustor at superatmospheric pressure.
In another aspect, the present invention provides a superatmospheric combustion apparatus that includes a superatmospheric combustion device having a lean gas chamber, a combustor, a heat recovery section, and an exhaust, a lean gas feed for feeding lean gas to the lean gas chamber, a heat sink/pressure equalization chamber and a preheated air chamber within the combustion device, a pressurized ambient air feed for feeding pressurized ambient air to the heat sink/pressure equalization chamber, a preheated air feed for feeding preheated air to the preheated air chamber, a lean gas port for feeding the lean gas from the lean gas chamber to the combustor, and a preheated air port for feeding the preheated air from the preheated air chamber to the combustor. The heat sink/pressure equalization chamber exchanges heat from the lean gas chamber, the preheated air chamber, and the combustor to the pressurized ambient air in the heat sink/pressure equalization chamber, and the lean gas and the preheated air are combusted in the combustor at superatmospheric pressure.
In the present invention, the preheated air chamber can be nested within the heat sink/pressure equalization chamber.
The present invention includes pressurizing the ambient air to a pressure of about 0.1 psig to about 10.0 psig, and more preferably, to a pressure of about 0.1 psig to about 5.0 psig.
The invention also includes preheating the preheated air to a temperature of about 200xc2x0 F. to about 1000xc2x0 F., and more preferably, to temperature of about 200xc2x0 F. to 600xc2x0 F.
The invention also can include increasing the temperature of the pressurized ambient air exiting the heat sink/pressure equalization chamber to a temperature of not more than about 500xc2x0 F., and more preferably, to a temperature of not more than about 300xc2x0 F.
The invention also can include a heat exchanger in the heat recovery section of the combustion device for preheating the preheated air, in which case, the pressurized ambient air can be discharged from the heat sink/pressure equalization chamber and fed to the heat exchanger. In another aspect, a heat source external to the combustion device can be used to preheat the preheated air, in which case the pressurized ambient air from the heat sink/pressure equalization chamber can be discharged and fed to the external heat source.
These and other features, objects and advantages of the invention will become apparent upon consideration of the following detailed description of the invention, the appended claims, and the several views of the invention, which are illustrated in the drawings.